Others are mentioning price per message, but also important to remember that in the 2000-2010s outside of the United States, when traveling from country to country, you usually had to get a local SIM card for a provider in that country, During your vacation, you’d have a new number for a month when no one could contact you over normal text messages which was a problem. WhatsApp became popular so you could keep chatting with friends abroad even if your phone’s SIM card changed during your vacation.

Since most people from the United States don’t travel outside the country, and vacation time is rather limited, this wasn’t a problem for them, but for the rest of the world, a solution was needed that removed the cell carrier from the equation.